An indirectly heated cathode, such as an indirectly heated cathode 100 shown in FIG. 7, is conventionally used as a cathode of an electron gun which is housed in a neck of a cathode ray tube.
As shown in the drawing, the indirectly heated cathode 100 includes a cylindrical cathode sleeve 102 enclosing a spiral heater 101, a caplike base 103 provided on the cathode sleeve 102, and an electron emitter layer 104 formed by coating an upper surface of the base 103 with an electron emitting material such as an alkaline earth metal using a spray or the like.
The cathode sleeve 102 is placed in a cylindrical cathode holder 105, and held by the cathode holder 105 via cathode supporters 106.
The cathode sleeve 102 has a function of transmitting heat generated from the heater 101 to the electron emitter layer 104. The cathode sleeve 102 is made of a material that contains nickel (Ni) and chromium (Cr) as main components.
When a cathode ray tube having this conventional indirectly heated cathode 100 is used for a longtime, however, the cathode sleeve 102 is significantly deformed by the heat generated from the heater 101. In particular, if the cathode sleeve 102 expands or contracts in the direction A shown in FIG. 7 and as a result its overall length changes, a distance between the base 103 and electrodes in the electron gun such as a control electrode changes, which causes a cutoff voltage to vary.
In cathode ray tubes, the cutoff voltage is an important parameter for setting the amount of electron beam emission. If the cutoff voltage varies, a proper image display cannot be achieved. Especially in the case of color cathode ray tubes that use three cathodes for R, G, and B, a color balance of a display image is greatly disturbed if the cutoff voltage in each cathode changes, which makes it impossible to properly display the image.
In view of this problem, an indirectly heated cathode in which a metal material of a cathode sleeve is formed into two or more layers of crystal structure is proposed in order to increase a resistance to heat deformation and thereby suppress variations in cutoff voltage (e.g. Japanese Patent Application Publication No. H09-102266).
This type of indirectly heated cathode, however, requires a complex manufacturing process since the crystal structure needs to be formed by repeating annealing and rolling. Besides, indirectly heated cathodes produced in separate furnaces may differ from each other. Furthermore, a sufficient resistance to heat deformation cannot be attained.
The present invention was conceived to solve the above problems, and aims to provide a reliable indirectly heated cathode that is easy to manufacture, suppresses variations in cutoff voltage by keeping a cathode sleeve from heat deformation caused by a long operation of a cathode ray tube, and exhibits little dispersion between products. The present invention also aims to provide a cathode ray tube having such an indirectly heated cathode.